1. Field of the Invention
This invention relates to reducing jet exhaust noise produced by a jet engine.
2. Description of the Prior Art
Jet engine noise around airports has always been a problem; but only recently has there been increased pressure on engine manufacturers to make their engines quieter, particularly at takeoff. One type of noise which contributes heavily to the overall noise of a jet engine is the noise created by the high velocity exhaust gases. This is a broad band noise covering a wide range of frequencies as opposed to noise created, for example, by compressor stages or by the burner which is noise of a relatively narrow frequency band. The latter type of noise may be reduced by using acoustic materials in the walls of the gas stream tuned to the narrow frequency range of concern. These acoustic materials have little effect on the broad band noise of the exhaust gases which are related almost wholly to the velocity of the exhaust gases.
A common type of engine today is the turbofan or bypass engine which includes an inner primary or core stream surrounded by an outer bypass stream. For conventional subsonic transports, the jet noise problem has been alleviated with the advent of the high bypass turbofan which has relatively large mass flow rates for the amount of thrust produced. These engines have low jet velocities, and hence, low jet noise.
Utilization of these high bypass turbofans in certain applications, such as a supersonic transport, may lead to unacceptable penalties in the total airplane performance and economics. For efficient operation, the supersonic transport requires engines of relatively low mass flow rate for the amount of thrust produced, resulting in high jet velocities and high jet noise. The engines considered appropriate for the supersonic transport are the turbojet and the low bypass ratio turbofan, which would then require jet noise suppression devices to reduce their jet noise to acceptable levels.
The supersonic transport engines of prior art usually operate at takeoff with a combination of bypass ratio, fan pressure ratio, turbine inlet temperature, and augmentation level (i.e. cycle selection) which yields a primary gas stream jet exhaust noise which is higher than the acceptable maximum total noise level within which the engine must operate. In some prior art designs, the bypass flow may be augmented, with the result that the bypass flow may also exceed the acceptable maximum noise level when the engine is operated at the required thrust level. In other prior art designs, the primary and bypass flow may be mixed and possibly augmented to some extent, but this results in the common flow exceeding the acceptable maximum noise level when operated at the required thrust level. In view of this, prior art engines for efficient supersonic transports were always required to include jet noise suppression apparatus in the primary stream, in both streams simultaneously or across the common exhaust of both streams to reduce the total noise to an acceptable level.
It is logical to ask why, in these prior art engines, higher bypass ratios or lower turbine inlet temperatures have not been used to lower the primary jet velocities and reduce the noise level of the primary gas stream, since increasing the bypass ratios and lowering the turbine inlet temperature does not significantly increase the noise level (or velocity) of the bypass stream. The reason for not having done this is because, although the noise level of the primary stream can be considerably reduced by this technique, the reduction in total engine exhaust noise level is not substantial and, in any event, it is not high enough to compensate for the performance losses at the higher bypass ratios or lower turbine inlet temperatures.